Power over Ethernet (PoE), which is outlined in IEEE Std 802.3™-2005 clause 33 (the PoE standard), refers to a technique for delivering power and data to an electronic device via Ethernet cabling. In a PoE system, power sourcing equipment (PSE) provides a power supply to electronic devices, which may be referred to as powered devices, via an Ethernet cable. PoE eliminates the need for a separate power source to deliver power to attached powered devices. Such powered devices may include voice over Internet protocol (VoIP) telephones, wireless routers, security devices, field devices to monitor process control parameters, data processors, and the like.
The PoE standard specifies that a PSE perform a powered device detection operation to determine whether the powered device is attached before supplying power via the Ethernet cable. To perform detection, the PSE provides a DC voltage (within a range of 2.8 to 10 Volts DC) on pairs of wires of the Ethernet cable and monitors a received current (Amps) or a received voltage (V) to detect a resistance within an expected range (e.g. between 19 and 26.5 K-ohms). The PSE determines the powered device's presence using a Volt-Amp (VA) slope related to the powered device's voltage/current signature. If the PSE does not detect a valid resistance, the PSE does not apply power to the Ethernet port assigned to the powered device.
Once a powered device has been detected, the PoE standard specifies that the PSE may optionally perform a power classification operation to determine power requirements of the detected powered device. The PoE standard specifies five device classes, classes 0-4, which define expected power consumption levels of powered devices. If the PSE supports power classification, the PSE applies a classification voltage (DC) to the Ethernet port associated with the detected powered device. Assuming that the powered device supports classification, the powered device applies a resistive load to attenuate the DC voltage, current, or any combination thereof, to produce a current signature for the device. The PSE determines the powered device's power classification based on this current signature.
For example, a powered device may draw a current to specify its classification. A current draw of zero to four mA corresponds to class 0, which is also the default class for devices that do not support classification. A current draw of 26 to 30 mA and of 36 to 44 mA corresponds to class 3 and class 4 devices, respectively. The PoE standard specifies that the PSE provide a power supply of approximately 15.4 watts to devices of class 0, class 3 and class 4. A current draw of between 9 and 12 mA corresponds to a class 1 device, and the PoE standard specifies that the PSE provide a power supply of up to approximately 4 watts to a class 1 device. A class 2 device corresponds to a current draw of 17 to 20 mA and requires the PSE to provide approximately 7 watts of power.
The PSE may use the powered device power classification to manage power allocation with respect to an overall power budget of the PSE. If a power level associated with the power classification of the powered device exceeds the available budget, the PSE need not apply power to the associated Ethernet port. If the power desired is within the power budget, the PSE may apply power to the associated Ethernet port. By utilizing power classification, a PSE may more accurately determine an associated power demand and may be able to support a larger number of powered devices than if the PSE reserved a maximum power consumption for each powered device.
In a strict interpretation of the PoE standard, a powered device that specifies a power classification that is less than the maximum should also limit its current consumption to be consistent with its classification. However, conventional powered devices set only a single current limit that is appropriate for its maximum powering mode. Therefore, there is a need for enhanced management of powered electronic devices.